Freshly isolated, mature dendritic cells (DC) from mouse lymphoid organs were analyzed by immunofluorescent labeling and flow cytometry to determine the number of discrete subpopulations and to assess possible lineage markers. The permanence of surface markers was then determined by overnight culture of the DC. Three DC subtypes were discerned, CD8alpha- DEC-205-, CD8alpha+ DEC-205+, and CD8alpha- DEC-205+, with different tissue distributions. The majority of DC expressed high levels of class II MHC, expressed CD11c, and expressed the costimulator molecules CD80, CD86, and CD40; CD80 and CD40 were further up-regulated on culture. DC also expressed low levels of L-selectin that were up-regulated on culture. Thymus contained predominantly CD8alpha+ DEC205+ CD11b- DC, resembling a major subpopulation of DC in other tissues but unique in expressing BP-1. Spleen contained predominantly two DC populations in equal proportions: one CD8alpha+ DEC-205+ CD11b- as in the thymus, and the other CD8alpha- DEC-205- CD11b+. Lymph nodes contained the same two DC populations as in spleen, but in addition a third population of CD8alpha- DEC-205+ CD11b- DC. The CD8alpha expression of splenic DC subpopulations did not change on culture. Although DEC-205 was up-regulated on culture so all DC became positive, the difference in the level between subpopulations was maintained. However, CD11b was up-regulated on culture, so all subpopulations became positive and finally expressed equivalent levels. Some aspects of this complex, but discrete, pattern of surface marker expression can be correlated with differences in lineage origin and functional activity of the DC.